The hub motors that are mostly used at present only rotate at the speed of the wheel, approx. 200 rpm at 25 km/h, and have to generate extremely high torque in order to produce a sufficient forward thrust at the outer circumference of the wheel. The material input, the weight and the costs of this design are correspondingly large. One additional disadvantage is that the considerable motor weight of approx. 3 kg contributes 100% to the unsprung mass and adversely affects the driving characteristics of spring-mounted wheels. Front-wheel motors can easily spin on a slippery road surface, resulting in a risk of crashing. Rear-wheel motors inhibit the installation of disc brakes as well as derailleur and hub gears.
Centrally mounted motors avoid these disadvantages because they are usually fitted directly behind the bottom bracket, have a highly reduced transmission ratio and transfer their power to the chain via a sprocket. However, the service intervals of these highly strained wearing parts are additionally shortened by the motor. Power flow is complicated and subject to high loss because the output of the high-revving motor first has to be transferred to the very low chain speed and from there reconverted to the high peripheral speed of the wheel. Mounting space and fastening of the motor have to be taken into account during the frame design, eliminating the possibility of retrospective installation.
One special variation of the centrally mounted motor circumvents this restriction because in this case the motor sits concentrically on the axis of the bottom bracket occupies the design space of the front sprocket and needs only one torque support on the frame. Consequently, it can be retrofitted without difficulty on many bicycles. However, compared with the hub motor, its weight problem is further aggravated. As it can only rotate at the pedaling frequency, 60 to 80 rpm, it has to generate almost three times as much torque.
Apart from the legendary VeloSolex with petrol engine, which has been successfully marketed in substantial quantities, four other applications are known from the patent literature that closely resemble the invention described here because they also pursue the aim of directly driving the wheel at its outer circumference by frictional contact from lightweight and high-revving motors. In physical terms, this path definitely makes sense because the performance capacity of electric motors increases proportionally with the rotational speed and, due to the high peripheral speed, a given output can be transferred with correspondingly smaller forces.
US 2011/0232985 most closely approximates the Solex because the motor, which is parallel to the axle, transfers its angular force directly to the tread of the rear tire.
OE 42 19 763 and OE 196 33 345 have a single motor positioned outside of or above the tire and drives both sides of the tire flanks via a transmission that also simultaneously receives the torque division on two friction rollers.
DE 690 31 993 [U.S. Pat. No. 5,078,227] does not need a transmission because here there are two counter-rotating motors also mounted outside/above the front tire and the friction rollers are located on the motor axles that extend downward.
Various arrangements of motor mount and levers enable the friction rollers to be actively moved into their operating position with a high contact pressure onto the tire or into a contact-free idling position.